Mercury in Tuna Sushi Higher at Restaurants than Groceries

Some species of tuna, particularly those that restaurants value for their firmer flesh and appealing look — such as bluefin akami — have higher mercury levels than other species typically found in grocery stores. Here, a plate of sushi with several pieces of tuna in the center (dark red is akami; light pink is toro).

Credit: Joanna Burger

Tuna sushi from your local supermarket might have lower mercury
levels and so be safer to eat than sushi from a high-end restaurant, a new
study using fish DNA suggests.

The results show that some species of tuna, particularly those
that restaurants value for their firmer flesh and appealing look — such as bluefinakami and all bigeye
tuna — have higher mercury levels than other species typically found in grocery
stores.

Overall, however, all the tuna had pretty high mercury
levels. The levels were, on average, greater than the concentrations considered
safe to consume in one day by the U.S. Environmental Protection Agency and
higher than the concentrations allowed in Japan.

The research could lead to better labeling to help tuna
eaters cut down on the amount of mercury they consume, something that can have
toxic effects, the researchers say.

"So far, the U.S. does not require restaurants and
merchants to clarify what species they are selling or trading, but species
names and clearer labeling would allow consumers to exercise greater control
over the level of mercury they imbibe," said study researcher Jacob
Lowenstein, a graduate student affiliated with the American Museum of Natural
History in New York, which conducted the genetic part of the research.

Sushi DNA

While previous studies have identified a difference in
mercury levels between tuna species, those studies weren't necessarily reliable
because they didn't have a fool-proof way of determining which tuna came from
which species, said study researcher Joanna Burger, a professor at Rutgers University.

The new study used a technique called DNA
barcoding to identify which tuna came from which species. With this method,
scientists use a specific DNA sequence from an organism — called a species
"fingerprint" — to match a sample of unknown origin with a certain
species. This comes in handy for identifying tuna species, since distributors
and supermarkets don't often know exactly what type of tuna they're selling, Burger
said.

"[This is the] first study where every sample of tuna
that we examined was barcoded so that we know for sure what the specie is,
there was no guess work," Burger said.

For now, consumers should be aware that that ordering tuna
sushi could come with risks.

The researchers took sushi
samples from 54 restaurants and 15 supermarkets in New York, New Jersey, and
Colorado. The samples were identified as being from as bigeye tuna, yellowfin
tuna or one of two bluefin tuna species.

Mercury levels in the bluefin akami species, (sushi
from lean, dark red tuna) and all bigeye tuna samples were significantly higher
than the levels in bluefin toro (sushi from fatty tuna) and yellowfin
tuna samples.

The average mercury levels in bluefin akami were higher than those permitted by the US
Food and Drug Administration (1 part per million, or ppm).

The varying mercury levels between the species are partly
due to the fact that mercury accumulates more in muscle than it does in fat,
leading to higher concentrations in the leaner bluefin akami.

However, yellowfin
tuna are also lean, but they might have lower mercury levels for a few reasons:
They are smaller than other tuna, don't eat as much, and tend to be killed at
younger ages, so they don't accumulate mercury for as long as other species.

The results were
published online today in the journal Biology Letters.

Health risks

Mercury is toxic to humans, and trace amounts can be found
in fish. The mercury levels increase as you go up the food chain, with
predatory fish generally having higher levels since they feed on smaller fish
and thus accumulate more mercury through their diet.

The element poses the greatest risk to unborn fetuses, since
their brains are still developing and their bodies are very small compared with
an adult. Exposure to high mercury levels in the womb can lead to cognitive
problems and learning disabilities later in life, according to Burger.

Women who are thinking about having children should also
watch out for high mercury levels, since the element can accumulate in muscle
tissue, Burger said.

Children and adults have been known to come down with
mercury poisoning, which can cause neurological and mobility problems.

Adults who get mercury poisoning like actor Jeremy Piven
in 2008, have probably eaten tuna steaks or swordfish two or three times a
week, or tuna sushi three or four times a week, according to Burger.

What to eat?

While there aren't any hard-and-fast rules when it comes to eating fish,
which has known health benefits, Burger recommends the following:

You could lower your exposure to mercury by eating
light tuna rather than white tuna, or albacore. In general, mercury levels in
albacore are three times higher than those found in light tuna.

Women who are considering becoming pregnant
should avoid fish with mercury levels greater than 0.5 ppm. This would mean not
eating tuna sushi, since you can't consistently count on getting tuna that is
on the lower end of the spectrum in terms of its mercury level.

Adults who are not pregnant and are not of childbearing
age shouldn't eat more than seven or eight pieces of tuna sushi per month.

Some fish such as salmon are generally lower in
mercury, and are safer to consumer in larger quantities.

In 2004, the FDA issued guidelines for pregnant women, women
of childbearing age and young children about how much fish to consume. The
report recommended this group eat no more than 12 ounces (two servings a week
on average) of canned light tuna and other types of fish, such as salmon and
catfish. The report recommended against eating any shark, swordfish, king
Mackerel, or tilefish because these fish tend to have higher mercury levels.

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.